text: Like in any other reactors, the hydrodynamics (or mixing) within a membrane bioreactor plays an important role in determining the pollutant removal and fouling control within the system. It has a substantial effect on energy usage and size requirements, and therefore the whole life cost of a membrane bioreactor is high.
  The removal of pollutants is greatly influenced by the length of time fluid elements spend in the membrane bioreactor (i.e. the residence time distribution). The residence time distribution is a description of the hydrodynamics of mixing in the system and it is determined by the design of the reactor (e.g. size, inlet/recycle flowrates, wall/baffle/mixer/aerator positioning, mixing energy input). An example of the effect of mixing is that a continuous stirred-tank reactor will not have as high pollutant conversion per unit volume of reactor as a plug flow reactor.
  The control of fouling, as previously mentioned, is primarily achieved via coarse bubble aeration. The distribution of bubbles around the membranes, the shear at the membrane surface for cake removal and the size of the bubble are greatly influenced by the hydrodynamics of the system. The mixing within the system can also influence the production of possible foulants. For example, vessels not completely mixed (i.e. plug flow reactors) are more susceptible to the effects of shock loads which may cause cell lysis and release of soluble microbial products.
  Many factors affect the hydrodynamics of wastewater processes and hence membrane bioreactors. These range from physical properties (e.g. mixture rheology and gas/liquid/solid density etc.) to fluid boundary conditions (e.g. inlet/outlet/recycle flow rates, baffle/mixer position etc.). However, some factors are peculiar to membrane bioreactors and these include the filtration tank design (e.g. membrane type, multiple outlets attributed to membranes, membrane packing density, membrane orientation, etc.) and its operation (e.g. membrane relaxation, membrane backflush, etc.).
  The mixing modeling and design techniques applied to membrane bioreactors are very similar to those used for conventional activated sludge systems. They include the relatively quick and easy compartmental modelling technique which will only derive the residence time distribution of a process (e.g. the reactor) or a process unit (e.g. the membrane filtration vessel) and which relies on broad assumptions of the mixing properties of each sub-unit. Computational fluid dynamics modeling, on the other hand, does not rely on broad assumptions about the mixing characteristics and instead attempts to predict the hydrodynamics from a fundamental level. It is applicable to all scales of fluid flow and can reveal much information about the mixing in a process, ranging from the residence time distribution to the shear profile on a membrane surface. A visualization of such modeling results is shown in the image.
  Investigations of membrane bioreactor hydrodynamics have occurred at many different scales ranging from examination of shear stress at the membrane surface to residence time distribution analysis for a complete membrane bioreactor. Cui et al. (2003) investigated the movement of Taylor bubbles through tubular membranes. Khosravi, M. (2007) examined an entire membrane filtration vessel using CFD and velocity measurements. Brannock et al. (2007) examined an entire MBR system using tracer study experiments and RTD analysis.